The present invention relates to composite preforms used in the formation of continuous fiber/metal matrix composites, and more particularly, to a method for retaining the composite preform elements in position during processing.
Composite preforms such as those used in the formation of continuous fiber/metal matrix composites are composed of an assembly of mono-filament fibers and a matrix metal in the form of foil, powder or wire. These assemblies may be used for the manufacture of a ring or a rod, for example. In each case, an assembly of precursor fibers and metal is rolled onto a mandrel to build up the reinforcement region. In a case of a ring, for example, this is accomplished by rolling up a collection of metal wires and fibers on an annular, recessed, substrate or mandrel to form the assembly. In the case of a rod, a pre-preg sheet is first fabricated consisting of a layer of fibers and a layer of metal wires bonded together with adhesive. This sheet is then rolled up onto a cylindrical mandrel to form the assembly.
In the case of both ring and rod assemblies, the rolled assembly must be held in place in such a way as to retain the relative positions of the fiber and metal elements throughout the fabrication process. In a case of the ring assembly, this requires holding the roll up from unwinding, and in the case of the rod assembly, this requires holding the roll-up from unwinding and accommodating shrinkage in the roll-up due to debulking which occurs in the rod roll-up during the off gas operation owing to the removal of the adhesive used to fabricate the pre-preg sheets.
Currently, the rolled assembly in organic composite fabrication is held in place through the use of an elastomeric bladder and an associated pressure differential that holds the bladder against the assembly. In the case of metal matrix composites, this processing hardware must be suitable for high temperature operations which are much higher in temperature than is suitable for the typical elastomer. Typically, therefore, the encapsulation hardware for such processing is composed of either steel or titanium metal, neither of which is elastomeric enough to be pushed against the rolled assembly by differential pressure until the process temperature and pressure have reached very high values. This results in a significant disadvantage for the reason that the assembly is in an unclamped state during most of the fabrication process which allows for unwanted movement of the metal and fiber assembly elements during processing.
Accordingly, a need has arisen for a simple and effective method for controlling the relative positions of the rolled assembly elements and clamping them in place throughout the entire fabrication process. The method of the present invention meets this need.
In accordance with the new and improved method of the present invention, the assembly of fibers and metal on the mandrel or substrate is clamped thereon and held in place during the entire fabrication process by winding over the assembly in a spiral fashion a wire formed of a suitable heat resistant metal such as a titanium. The ends of the clamping wire may be fixed by inserting them in grooves or other apertures in the mandrel or substrate, or by otherwise securing them to the mandrel or substrate. The pitch of the winding and the tension applied by the clamping wire during the spiral winding thereof is selected based on the specific roll-up assembly characteristics. In the case of a metal powder/fiber assembly, for example, the clamping wire overwrap is wound tight to itself, i.e., such that the overwrap wires are in engagement with each other. This eliminates the migration of the power from the roll-up assembly during processing. In the case of a metal wire/fiber assembly, or a metal foil/fiber assembly, the spacing between the clamping wires can be greater.
In this manner, tension on the clamping wire during winding assures intimate contact between the wire overwrap and the rolled assembly as well as establishing a certain amount of elastic compliance to the overwrap. The clamping wire overwrap may be tensioned during winding, for example, to accommodate for a predetermined diameter contraction of the roll-up assembly during processing.